In the semiconductor lasers, the distributed feedback semiconductor laser that is provided in the direction of advance of a waveguide with periodic structures for refractivity and gain will discharge a very important role as a device for use in the future Wavelength Division Multiplexing (WDM) operation in respect that it is capable of controlling an oscillating wavelength exactly and it facilitates integration because it obviates the necessity of a step of cleavage indispensable as in the Fabry-Perot laser.
The first problem that is encountered by the distributed feedback semiconductor laser pertains to simplification of the process of production thereof In the initial stage of its development, it was customary to form a lower clad layer, a lower guide layer, an active layer and an upper guide layer from a substrate upward in the first cycle of the process of crystal growth, curve gratings conforming to the wavelength in a waveguide on the upper guide layer, and form an upper clad layer on a guide layer having a periodic structure in the second cycle of the crystal growth (refer, for example, to Document 1: P. K. York, J. C. Connolly et al, “MOCVD regrowth over GaAs/AlGaAs gratings for high power long-lived InGaAs/AlGaAs lasers,” Journal of Crystal Growth 124 (1992) 709-715).
Further, for the purpose of securing the confinement of carriers and optical field in the lateral direction, stripes are formed along an optical waveguide by means of wet etching through a silicon dioxide mark, and lateral surfaces of the stripes are embedded with a current blocking layer and an ohmic contact layer by the third cycle of growth. It is often the case that the fourth cycle of growth is necessary to flatten the device surface after removing the silicon dioxide mark used for the third selective growth.
Such multiple cycles of lithography and crystal growth results in boosting the cost of production and impeding the dissemination of the product in industry. Further, since the re-growth interface exists in the neighborhood of the active layer, it causes an additional leak path of the drive current and increases the threshold current. The technique of this nature, therefore, has its limits in terms of principle and cannot continue to be similarly useful in the future.
In contrast, the quantum nano-structure semiconductor laser using quantum wires and quantum dots in the active domain has been found to possess various great merits besides the quantum effect initially expected. In the case of the quantum dots, for example, particularly since the dots are embedded with a material of a broad band gap, they can confine therein a carrier and, by virtue of a simple alteration of the conditions for crystal growth, can aim at attaining an addition to the function without entailing an increase in the cost of production. The merit of this nature suggests the possibility of proving extremely advantageous for the construction of semiconductor lasers that will find utility fiber to home age to arrive shortly.
The present inventors, therefore, have hitherto studied processes for the production of quantum wires and have searched stepwise for new processes to redue the device production cost while maintaining the device performance. Generally, the quantum nano-structure such as a quantum wire or quantum dots with a narrow band gap material embedded in a wide band gap material to form a clad layer in a size of several nm equaling the de Broglie wavelength of an electron has a density of stress concentrated on a specific energy level and, therefore, befits the realization of a high performance optical devices. Even for the sake of clearing the problem regarding a process complexity of device production, therefore, it is more rational to aim at realizing such a quantum nanostructure configuration. That is, in having quantum wires integrated at a high density in a positional relation of a specific regularity, it is ideal to realize this integration by one time crystal growth. The realization of this integration results in rational realization of a semiconductor laser allowing control of wavelength and a supersaturated absorber necessary for self-starting oscillation of an ultrahigh speed solid laser.
For the sake of confining light in a semiconductor waveguide, it is necessary that the upper and lower clad layers should be formed at least in a thickness in the approximate range of 0.5 to 1 μm. When a grating is formed on a substrate and a lower clad layer of such a thickness is grown and, even thereafter, the grating is allowed to retain a satisfactory shape on the surface of the lower clad layer, then it is made possible to form an active layer approximating closely to the grating of the clad layer by one time crystal growth and consequently simplify a process for the fabrication of a distributed feedback semiconductor laser prominently.
From this point of view, the present inventors have suggested a method of forming falcate quantum wires by first forming stripe patterns in the (1-10) direction on a compound semiconductor substrate of the (100) azimuth by following the procedure reported in Document 2 (Xue-Lun Wang et al., “Fabrication of highly uniform AlGaAs/GaAs quantum wire superlattices by flow rate modulation epitaxy on V-grooved substrates,” Journal of Crystal Growth 171 (1997) 341-348), forming V-grooves by wet etching, growing thereon AlGaAs and InAlAs confining in the respective compositions Al, an element sparingly producing surface atomic migration, thereby forming a clad layer retaining the profile of V-shaped grooves, and then supplying GaAs and InGaAs confining Ga and In, elements producing a large surface atomic migration. In this case, when (111)A planes are allowed to be formed as inclined planes to intersect each other, it is made possible to attain growth to a thickness of 1 μm or more in the direction of thickness of growth while the shape of V-grooves mentioned above is retained satisfactorily by setting a proper temperature of crystal growth so as to suit the crystal mixing ratio of the compound semiconductor.
The stationary growth profile is made possible when the growth rate of the (111)A plane having a lower rate of crystal growth against that of the (100) plane having a higher rate of growth is equal to sin θ relative to the inclination θ of the crystal plane. Generally, the rate of growth of a specific plane depends on the chemical activity of that plane and the diffusion of the raw material elements from the environment, and the anisotropy due to the azimuth of a crystal plane tends to fade and the rate of growth becomes uniform in accordance as the temperature increases. The rate of growth declines in the azimuth of inactive crystal such as the (111)A plane when the temperature decreases. By adjusting the temperature of the substrate, therefore, it is made possible to form grating profiles at a fixed period.
In this suggestion, however, the period of repetition, namely the pitch of parallel V-grooves, is restricted to the order of microns. This pitch proves unduly coarse as for the purpose of producing a distributed feedback semiconductor laser of fully satisfactory characteristic properties and requires further refinement to the order of submicrons. In the case of submicron gratings having the shortest possible period relative to the distance of diffusion of Ga atoms adhering to the surface of the substrate, however, it is generally considered difficult to attain the necessary growth while a specific profile of crystal growth is retained. In fact, the growth was impossible at first.
Subsequently, the present inventors, as a result of further experiments and studies, managed to succeed in satisfactorily retaining the profile of V-grooves even on the surface of an AlGaAs layer grown on a substrate, though to a certain thickness.
This achievement is reported in Document 3 (C. S. Son, T. G. Kim, X. L. Wang and M. Ogura, “Constant growth of V-groove AlGaAs/GaAs multilayers on submicron gratings for complex optical devices,” J. Cryst, Growth, Vol. 221, No. 1/4, pp. 201-207 (December 2000)).
In finding the maximum film thickness of the AlGaAs layer formed on the GaAs substrate gratings while the profile of the gratings is infallibly retained, a trial of alternate superposition of AlGaAs layers about 100 nm in thickness and GaAs layers about 10 nm in thickness will facilitate due judgment. Document 3 mentioned above inserts a report regarding trial alternate superposition of a pair of an AlGaAs layer having a relatively large thickness of about 100 nm and a GaAs layer having a relatively small thickness of about 10 nm on a GaAs substrate having V-grooves formed with a pitch of 0.38 μm on the surface thereof As a result of this experiment, it was found that the profile of the V-grooves of the substrate was satisfactorily retained up to about 1 μm in thickness of superposed layers from the surface of the substrate. A GaAs quantum wire was formed in a falcate cross section and in parallel to the bottom parts of these V-grooves. According to the technique prevalent at the time of disclosure of this publication, however, the profile of V-grooves was seriously impaired when the height of superposed layers reached a level exceeding 1 μm.
Of course, in the actual manufacture such as of a distributed feedback semiconductor laser, though one AlGaAs layer suffices as a clad layer and one or more GaAs quantum wires laid in the vertical direction suffice, it may be safely concluded that the lower V-grooves have a better profile and the quantum wires formed in these V-grooves likewise have a better cross-sectional shape in accordance as the upper V-grooves offer more resistance to the collapse of profile. This fact proves that even on the upper surface of a single AlGaAs clad layer formed in an arbitrary film thickness, gratings are enabled to retain a fully satisfactory profile and implies that the quantum wires to be formed thereon are similarly satisfactory. Even an active layer appearing to be a quantum well layer of the shape of a continuous plane and not quantum wires, namely even an active layer of the shape of a fairly uniform flat plane (the shape of a sheet) having the thickness and width thereof not geometrically modulated or corrugated in conformity with the period of gratings of V-grooves, allows the periodic structure such as of the distribution of refractivity, supposed that gratings of either a guiding or cladding layer underneath are constructed with such high accuracy as expected, and can be similarly utilized very effectively as an active layer in a distributed feedback type semiconductor layer. For the sake of simplicity, the quantum wires will be exclusively described below.
The present inventors have further made studies and experiments with a view to enabling the V-grooves up to a greater thickness of superposed layers to retain a good profile and consequently have succeeded in improving the technique disclosed in Document 3 mentioned above to an extent of suggesting such conditions that even when the clad layer is formed in a thickness at least exceeding 1 μm, preferably approximating closely to or even surpassing 1.5 μm, the V-grooves formed on the surface thereof may retain a fully satisfactory profile. The invention perfected based on this knowledge has been already disclosed in Japan Patent Application No. 2000-404645 (JP-A 2002-204033).
In this patent document, a basic structure is reported to be obtained by etching a plurality of V-grooves extending in the [01-1] direction on a (100) GaAs substrate with a pitch of the order of submicrons in such a manner that the lateral surfaces thereof each constitute a (111)A plane, subjecting the V-grooves to a treatment for removal of a surface oxide layer, thereby enabling the V-grooves to retain an angle of 80 degrees or less even after the treatment, and thermally cleaning them at a temperature in the range of 680° C. to 720° C., thereby forming on the surface of the GaAs substrate a buffer layer of the same material GaAs. These treatments enable the apexes between the adjoining V-grooves which have been dulled by the thermal cleaning to be recovered, allow an AlGaAs layer having an Al percentage of 0.3 to 0.6 or an InAlAs layer having an In percentage of 0.05 to 0.3 to be grown as a clad layer, and further warrant supply of GaAs or InGaAs as well.
Further, the process of growing on the part forming quantum wires or a quantum well layer an AlGaAs guide layer having a smaller Al percentage than the Al percentage of the AlGaAs layer constituting a clad layer or an InAlAs guide layer having a smaller In percentage than the In percentage of the InAlAs layer constituting a clad layer and growing further thereon as an upper side clad layer an AlGaAs layer having an Al percentage of 0.3 to 0.6 or an InAlAs layer having an In percentage of 0.05 to 0.3, is actually favorable for the manufacture of a device utilizing this invention.
By this technique, the V-grooves are enabled to retain a fully satisfactory profile till the height of the laminated structure generously exceeds 1 μm and even reaches 1.5 μm. Of course, the fact that the profile of V-grooves can be retained to such a height proves that the profile of quantum wires in the lower part and the profile inherent in the V-grooves are highly favorable. In fact, when this technique is embodied in quantum wires embedded in an active layer of a distributed feedback semiconductor laser, the quantum wires are found to acquire more than satisfactory characteristic properties. In short, the invention of the aforementioned Japanese Patent Application has established that even when an AlGaAs layer is grown as a single clad layer to a thickness in the range mentioned above by way of an experiment of constructing a laminate structure formed by repeating such multilayer films, the profile of gratings formed on the surface thereof match the substrate gratings and can be retained fully satisfactorily. The quantum wires that are formed thereon acquire fully satisfactory profile and characteristic properties as a matter of course. The AlGaAs clad layer having a thickness falling short of the upper limit of the range can be expected to bring still better results.
As regards the quantum wires, the present inventors' efforts have developed such an environment as allows provision of quantum wires of fairly better performance. The method of production thereof is simple and capable of forming by one time selective growth of high-density multiple quantum wires with highly satisfactory profile and characteristic properties at a necessary position in the structure of a given device. Such an excellent structure of quantum wires as this has not made a true contribution to the industry unless it has achieved a development in terms of application.
This invention has been initiated with a view to developing such applications and, therefore, is aimed at providing a quantum nano-structure semiconductor laser particularly promising a growing demand, which is capable of satisfying at least either or preferably both of the reduction of the threshold value and the stabilization of frequency of oscillation, the factors which are constantly in need of improvement. It is provided, however, that with the same intent, this invention is not limited to the quantum nano-structure semiconductor laser but has as its intrinsic object the provision of a quantum nano-structure array which, uses periodically disposed limited-length quantum wires or quantum dots and which can be developed into various optically functioning devices.